1. Field of the Invention
The present invention relates to a telemetry system for digital transmission of data. More particularly, the present invention relates to a system and method using amplitude modulation of Walsh functions for data transmission, thereby simplifying the system design.
2. Description of the Related Art
Information in digital form possesses many advantages over information in analog form. For example, information in digital form is less easily corrupted and more easily transformed than information in analog form. It is precisely these advantages that make the digital form desirable for information communication. Digital communication has evolved into a science and an industry. It is ubiquitous in our everyday world. Telephone systems, satellite television, compact disks (CDs), hard drives, and computer networks each rely on the principles of digital comnmunication.
Pulse amplitude modulation is a well-established technique of digital communications in which a sinusoidal carrier signal is modulated to one of two amplitude levels, corresponding to one of two in binary values. Multiple amplitude modulation is a similar technique in which the sinusoidal carrier is modulated to one of multiple discrete amplitude levels, each of which corresponds to one of multiple possible values. Quadrature amplitude modulation (QAM) is yet another established technique. QAM uses two sinusoidal carriers with that have the same frequency, but are 90 degrees out of phase. Because these carriers are orthogonal, they can each be independently modulated to one of multiple discrete amplitude levels. This significantly increases the amount of information that can be communicated in a given time interval. Details on these techniques can be found in many standard digital communications textbooks including, for example, Proakis, J. G., Digital Communications, 2/e, McGraw-Hill Book Company, New York, 1989.
While sinusoidal carriers may have some advantages, there exist other orthogonal waveforms which may also prove advantageous. FIG. 1 shows a sample of one such class of waveforms known as Walsh functions (Walsh, J. L., “A closed set of orthogonal fuinctions”, American Journal of Math., vol.55, pp. 5-24, 1923). These functions have the desirable property that they are bipolar, i.e. the amplitude of each fuinction is either +1 or −1, and have applications as discussed by H. E. Harmuth, in “Applications of Walsh fuinctions in communications”, IEEE Spectrum 1969.
As can be seen from FIG. 1, inside a basic interval β from −½ to +½, the Walsh functions only take on 2 values, +1 and −1. Outside this interval the functions are zero. The odd functions of this series are labeled sal(i,β), where i is the “sequency” or “order” of the sal function. The order of the function is related to the number of zero crossings in the function, in that the number of zero crossings is 2x the order of the fuinction. The even functions of this series are termed cal(i,β) where i again is the order defined in the same way. Because each of the Walsh fuinctions are bi-valued, they are easy to generate using digital circuitry. Each Walsh function is characterized by order rather than by frequency.
Because Walsh functions are easily generated using digital circuitry, a desirable reduction in system complexity may be achieved by designing digital communications systems to exploit the properties of Walsh functions. This reduction in complexity, if accompanied by a consequent increase in reliability, may be particularly desirable for remote telemetry systems.